Science (English Medium) Class 12 - CBSE Question Bank Solutions for Physics

The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 3 m away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose?

A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens.

1. Determine the ‘effective focal length’ of the combination of the two lenses, if they are placed 8.0 cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notion of the effective focal length of this system useful at all?
2. An object 1.5 cm in size is placed on the side of the convex lens in the arrangement (a) above. The distance between the object and the convex lens is 40 cm. Determine the magnification produced by the two-lens system and the size of the image.

An object 1.5 cm in size is placed on the side of the convex lens in the arrangement (a) above. The distance between the object and the convex lens is 40 cm. Determine the magnification produced by the two-lens system, and the size of the image

A man with normal near point (25 cm) reads a book with small print using a magnifying glass: a thin convex lens of focal length 5 cm.

(a) What is the closest and the farthest distance at which he should keep the lens from the page so that he can read the book when viewing through the magnifying glass?

(b) What is the maximum and the minimum angular magnification (magnifying power) possible using the above simple microscope?

A card sheet divided into squares each of size 1 mm² is being viewed at a distance of 9 cm through a magnifying glass (a converging lens of focal length 9 cm) held close to the eye.

1. What is the magnification produced by the lens? How much is the area of each square in the virtual image?
2. What is the angular magnification (magnifying power) of the lens?
3. Is the magnification in (a) equal to the magnifying power in (b)? Explain.

Figure shows an equiconvex lens (of refractive index 1.50) in contact with a liquid layer on top of a plane mirror. A small needle with its tip on the principal axis is moved along the axis until its inverted image is found at the position of the needle. The distance of the needle from the lens is measured to be 45.0 cm. The liquid is removed and the experiment is repeated. The new distance is measured to be 30.0 cm. What is the refractive index of the liquid?

1. Using the Bohr’s model, calculate the speed of the electron in a hydrogen atom in the n = 1, 2 and 3 levels.
2. Calculate the orbital period in each of these levels.

The radius of the innermost electron orbit of a hydrogen atom is 5.3 × 10⁻¹¹ m. What are the radii of the n = 2 and n = 3 orbits?

In accordance with the Bohr’s model, find the quantum number that characterises the earth’s revolution around the sun in an orbit of radius 1.5 × 10¹¹ m with orbital speed 3 × 10⁴ m/s. (Mass of earth = 6.0 × 10²⁴ kg)

The gravitational attraction between electron and proton in a hydrogen atom is weaker than the Coulomb attraction by a factor of about 10⁻⁴⁰. An alternative way of looking at this fact is to estimate the radius of the first Bohr orbit of a hydrogen atom if the electron and proton were bound by gravitational attraction. You will find the answer interesting.

Obtain the binding energy (in MeV) of a nitrogen nucleus `(""_7^14"N")`, given `"m"(""_7^14"N")` = 14.00307 u.

Obtain the binding energy of the nuclei `""_26^56"Fe"` and `""_83^209"Bi"` in units of MeV from the following data:

`"m"(""_26^56"Fe")` = 55.934939 u

`"m"(""_83^209"Bi")`= 208.980388 u

The neutron separation energy is defined as the energy required to remove a neutron from the nucleus. Obtain the neutron separation energies of the nuclei `""_20^41"Ca"` and `""_13^27 "Al"` from the following data:

`"m"(""_20^40"Ca")` = 39.962591 u

`"m"(""_20^41"Ca")` = 40.962278 u

`"m"(""_13^26"Al")` = 25.986895 u

`"m"(""_13^27"Al")` = 26.981541 u

Consider the fission of `""_92^238"U"` by fast neutrons. In one fission event, no neutrons are emitted and the final end products, after the beta decay of the primary fragments, are `""_58^140"Ce"` and `""_44^99"Ru"`. Calculate Q for this fission process. The relevant atomic and particle masses are

`"m"(""_92^238"U")` = 238.05079 u

`"m"(""_58^140"Ce")` = 139.90543 u

`"m"(""_44^99"Ru")` = 98.90594 u

In an intrinsic semiconductor the energy gap E_gis 1.2 eV. Its hole mobility is much smaller than electron mobility and independent of temperature. What is the ratio between conductivity at 600K and that at 300K? Assume that the temperature dependence of intrinsic carrier concentration n_iis given by

`"n"_"i" = "n"_0 exp(- "E"_"g"/(2"k"_"BT"))`

where n₀ is a constant.

State Bohr's postulate to define stable orbits in the hydrogen atom. How does de Broglie's hypothesis explain the stability of these orbits?

Using Bohr's postulates, derive the expression for the orbital period of the electron moving in the n^th orbit of hydrogen atom ?

State Bohr postulate of hydrogen atom that gives the relationship for the frequency of emitted photon in a transition.

Plot a graph showing the variation of photoelectric current with intensity of light. The work function for the following metals is given:
Na: 2.75 eV and Mo : 4.175 eV.
Which of these will not give photoelectron emission from a radiation of wavelength 3300

\[A^\circ\] from a laser beam? What happens if the source of laser beam is brought closer?